This core will provide Projects 1, 2 and 3 as well as the Antibody Core with necessary influenza virus reagents to conduct the proposed research. All three Projects as well as the Antibody Core will greatly benefit from this core's ability to produce a large panel of high quality recombinant influenza virus hemagglutinin and neuraminidase proteins as well as chimeric versions of these molecules. These proteins are functional, correctly folded and glycosylated and will be used in various assays by Project 1. 2 and 3 and as baiting agent by the Antibody Core. Furthermore, the core will produce custom made recombinant viruses that express chimeric hemagglutinin, neuraminidase and chimeric neuraminidase proteins in the influenza A and B virus background. Project 1 will use these viruses for analysis of monoclonal antibodies and for mimicking of preexisting immunity in animal models. Finally, the Reagent Core will supply Projects 1, 2 and 3 as well as the Antibody Core with high titer virus stocks, mouse adapted influenza virus challenge strains and purified virus preparations